This invention is related to active matrix liquid crystal display systems.
In conventional amorphous silicon thin film transistor liquid crystal displays (TFTLCD), the fabrication of the active matrix requires at least six to nine masking steps. For a such large number of masking steps, the production yield is low and the cost is high.
F. Morin et al suggested a low cost manufacturing method for the TFTLCD. Two masks were used for fabricating the TFT array. The method was published in the paper, "An Improved Design of Active Matrix LCD Requiring Only Two photolithographic Steps", at the 1985 International Display Research Conference, p. 34. F. Maurie et al further improved the method, as disclosed in U.S. Pat. No. 4,783,147, "Active Matrix Display Screen Without Spurious Transistor".
These two kinds of TFT have a top-gate transistor structure. H. C. Huan published a paper in the IEEE Electron Device Letters, "Dual-Gate a-Si:H Thin Film Transistors" vol EDL-3, no. 12, p. 357, proving that the top-gate a:Si:H TFT is inferior to bottom gate TFT in characteristics. The shortcomings are as follows:
1. The gate dielectric layer of silicon nitride, a-SiN, is deposited after the deposition of the semiconductor layer of amorphous silicon a-Si, the mobility of the TFT thus constructed is lower than that for the inverted staggered a-si TFT with the gate placed at the bottom of the structure.
2. When the semiconductor layer is directly in contact with the glass substrate, the semiconductor-glass interface adversely affects the characteristic of the TFT. The characteristic of the TFT with a top gate is unstable.
When the top-gate TFT is used for TFTLCD, there are further drawbacks:
1. As shown in FIG. 1, the gate electrode and the black matrix (for shielding light) are both placed above the semiconductor layer. Light from the substrate side is directly incident on the semiconductor layer, thus increasing the leakage current due to photoelectric effect.
2. The data line and the picture element (pixel) are of the same transparent conductive indium-tin-oxide(ITO) layer. If the distance between them is small (for decreasing the pixel area), it is very easy to cause a short circuit or a bridge, and produce a point defect. The problem is particularly serious for manufacturing large area TFTLCDs. The resolution of the aligner for exposing TFTLCDs decreases as the exposed area is enlarged, thus increasing the likehood of the bridge effect.
3. When ITC is used as data lines, the data line resistivity is excessive. This large resistivity limits the size of the LCD panel, because a large RC time constant slows down the frequency response, which, in turn, degrades the grey scale of the display.